E^ON RESEARCH AND ENGINEERING COMPANY

/ t

a* a* CO

PHOTODISSOCIATION DYE LASER

Semiannual Technical Report

April 1976

Contract Period Covered: October 1, 1975 - March 31, 1976

ARPA Order Number Program Code Contractor Effective Date of Contract Contract Ecpiration Date Amount of Contract Contract Number Principle Investigator Scientific Officer Short Title of Work

16

Engineering Company

1806, Amendment 6E 20 Exxon Research & October 1, 1974 August 15, 1976 $199,997 N00014-73-C-0048 Abraham Kasdan (201-474-3947) Director, Physics Program ONR Photodissociation Dye Lasf»r

The views and conclusions contained in this document are those of the author and should not be interpreted as urcessarily representing the official policies, either expressed or implied, of the Advanced Rese irch Projects Agency or the U.S. Government

Sponsored By:

Advanced Research Projects Agency ARPA Order No. 1806 n D C

EXXON/GRUS.3BE0B.76

BOX 8 LINDEN, NEW JERSEY 07036

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111 mmmmmmmmßiKm ' I '"■ ' ■'- ■ ~^. -' " ».

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UNCLASSIFIED «fCUWMV Cl »SMMT »1ION Of TM!» P«r,r ClfN^o />•!• I nlrwrm EJgON

RFPORT DOCUMENTATION PAGE 2. OOVT ACCESSION NO

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MONITORINO ACEMCY NAME » AOCRE SSrd di/lrr^-il frcm t anliolllnt OIO,

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'2

HKAD INSTRUCTIONS nFFORF. COMPUKTINf, IOHM

I RECH'IENT'S CATALOG NUMBER

» TYPE Of NCPONT • MNtOB COVrRCD

Oct. 1, 1975^arch 31, 1976

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NOt)Ol4-73-C-O04^

ARf>^5pRK UNIT NUMBERS

ARPA Otder No. 4806, Amend. 16--Program Codfe No. 6E 20

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T ^iKoanniA BI technical' rept 17. BfITIWÄUTION «TATCMtHT f«lrti« «In»»?» ••«•»<»" »l»c» ». M «M«»««» «r«'" "»po'O / ^ _, / -, , -^7 y^ a j

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I» KEY «OROS fConfMU« on ffitr «rd» 1/ 1

Photodissociation Tunable Liquid Lasers

«f( »nd itientltr hf blot* numfc»rj

20 AB'J ÖTSACT he photodissociation dye laser (PDL) program is an effort to develop

a new class of tunable liquid lasers operating in the visible region of the spectrum. In the PDL scheme, laser action is expected to occur between an excited electronic state and the ground electronic state of radicals produced by the photodissociation of specific classes of molecules in solution. This scheme is expected to result in several significant advantages over

conventional dye lasers

EJfcON 1473 EDITION OE

i

-*E I NOV »» ls\)SSOLtTE UNCLASSIFIED ^ '"f / SECURITY CLASSIEICATIOH OF TMlS (»AOt (»hmn P.I« Bf 1«»^

b

UNCLASSIFIED HCUWITY CcM'.lHi»noN Of IHH PXHim»»* Hal* KnirnJ)

During this reporting period we have completed our measurements on the potential PDL molecules that were selected from the hexaarylethane group, and have evaluated their use as active media for the PDL scheme.

Fluorescence intensity measurements as a function of excitation wavelength have been made on the hexaphenylethane-triphenylmethyl radical system. The measuretr its indicate that the optimum excitation source for the system is the N2 laser. However, because the maximum gain is low (~.02 cm"1), the length of active media the N2 laser is capable of exciting is too short to overcome the single pass cavity losses. Xenon flashlamp pumping of this system has been considered, but the high intensities need (>10 MW in the UV) makes such a pumping scheme unattractive.

In addition to hexaphenylethane, the other molecules that were synthesized in the early part of the program have been investigated. In particular, fluorescence spectra and lifetimes of pentaphenylethane, 1-p-biphenyl-l, 1,2,2-tetraphenylethane, and 12-12' bifluoradenyl have been measured. These molecules have fluorescence lifetimes of ~10 ns. However, they exhibit no significant photodissociation into excited state radicals when excited in the UV (2600 Ä).

■

BY..

"1 .

UNCLASSIFIED SCCUNITV CLAIIIFICATION OF TMI« *»Otfl«>"i Omlm *»r*r*tf)

'"'umiilWfvtfm.-v'*—(5«%_

TABLE OF CONTENTS

Page

I . INTRODUCTION i

IT. THE PHOTODTSSOCIATION DYE LASER CONCEPT 4

[II. PROPERT1 ES OF THE SELECTED PDL MOLECULES 8

A. Hexaphenyle thane 8 H. Other Hexaarylethane Derivatives 12

1V. EXPERTMENTAL EFFORT AND RESULTS 17

A. Hexaphenylethane 17 B. Other PDL Molecules 22

(a) Pentaphenylethane 22

(b) 1-p-biphenyl-l, 1,2,2-tetraphenylethane 22 (C) 12-12' bifluoradenyl 25 (d) Triphenylmethyl chloride 25

V. CONCLUSIONS 27

VI. REFERENCES 28

—"- ■ —" -'

Figure

1

5

6

LIST OF ILLUSTRATIONS

Photodissociation Dye Laser Molecule-- Structure and Kinetics

Energy Level Structure of the Hexaphenylethane— Triphenylmethyl System

Mirror Image Symmetry of the Absorption and Fluorescence Spectra of the Triphenylmethyl Radical (Low Temperature Data)

Hexaphenylethane Fluorescence Intensity as a Function of Excitation Wavelength

UV Absorption Spectrum of Hexaphenylethane

Fluorescence Spectrum of Pentaphenylethane

Fluorescence Spectrum of 1-p-l,1,2,2 Tetraphenyl- e thane

Fluorescence Spectrum of 12-12' Bifluoradenyl.

Page

6

13

14

19

20

23

24

26

ABSTRACT

The photodissociation dye laser (PDL) program is an effort to

develop a new class of tunable liquid lasers operating in the visible

region of the spectrum. In the PDL scheme, laser action is expected to

occur between an excited electronic state and the ground electronic

state of radicals produced by the photodissociation of specific classes

of molecules in solution. This scheme is expected to result in several

significant advantages over conventional dye lasers.

During this reporting period we have completed our measurements

on the potential PDL molecules that were selected from the hexaarylethane

group, and have evaluated their use as active media for the PDL scheme.

Fluorescence intensity measurements as a function of excitation

wavelength have been made on tne hexaphenylethane-triphenylmethyl radical

system. The measurements indicate that the optimum excitation source

for the system is the N? laser. However, because the maximum gain

is low (~.02 cm" ), the length of active media the N2 laser is "apable

of exciting is too short to overcome the single pass cavity losses.

Xenon flashlamp pumping of this system has been considered, but the

high intensities need (> 10 MW in the UV) makes such a pumping scheme

unattractive.

In addition to hexaphenylethane, the other molecules that were

synthesized in the early part of the program have been Investigated.

In particular, fluorescence spectra and lifetimes of pentaphenylethane,

1-p-biphenyl-l,1,2,2-tetraphenylethane, and 12-12" bifluoradenyl have

been measured. These molecules have fluorescence lifetimes of ~10 ns.

However, they exhibit no significant photodissociation into excited

state radicals when excited in the UV (2600 A).

'^sßf^mif^v^Fi^'-

INTRODUCTION

The photodissociation dye laser (PDL) program is an effort to

develop a new class of tunable liquid lasers operating in the visible region

of the spectrum. In the PDL scheme, laser action occurs between an excited

electronic state and the ground electronic state of radicals produced by the

photodissociation of specific classes of molecules in solution. In this

study, a class of highly strained ethanes, the hexaarylethanes, is being

investigated as potential laser media. Continuous wave operation of such

lasers is expected based on the rapid relaxation of the terminal vibronic

levels and the subsequent depletion of the electronic ground state by recombina-

tion to the original parent ethane structure. A? in conventional dye laser

systems, wavelength tunability is assured because the numerous vibrational

and rotational modes of the complex molecule result in a manifold of quasi-

continuous vibronic energy levels associated with each electronic level.

The photodissociation dye laser is expected to exhibit several

significant advantages over conventional dye lasers. The radical can be

thought of, to a first approximation, as a one-electron system. As a result,

the electronic structure of the radical consists of doublet levels rather

than the singlet and triplet level structure characteristic of the paired

electrons of conventional dye molecules. Consequently, problems associated

with non-radiative intersystem-crossing transitions which result in deleterious

absorption by the lowest triplet level at the laser wavelength in conventional

dye molecules are eliminated.

The second advantage of the PDL scheme is related to the upper

laser level lifetime. In conventional laser dyes, the spontaneous radiative

lifetime of the first excited singlet level is typically on the order of a

2 -

few nanoseconds. In radical systems, however, there is evidence, both

theoretical anü experimental, that the first doublet-doublet electronic

transitions are in some cases, partially forbidden. From the point of view

of the PDL scheme, this means that the radiative lifetime of the upper laser

level can be one to two orders of magnitude longer than that exhibited by the

upper laser levels of conventional organic dyes. Consequently, a larger

population density in the upper level may be obtained, thereby allowing the

possibility of generating higher laser output energy than can be presently

attained with conventional dyes. This assumes that strong absorption from

the upper level at the laser wavelength does not occur. In addition, the

longer radiative lifetime favors the recombination of the radical ground

state to the parent dimer at a sufficiently fast rate to permit a continuoas

population inversion and hence, cw laser action.

The aim of the current program is to demonstrate proof of

principle of the PDL scheme. Towards that end, a number of promising

chemical candidates from the hexaarylethane group of organic molecules

have been synthesized and data has been obtained on those processes

that are relevant to assessing their usefuli,«»ss as PDL laser mer'ta.

During the present reporting period we have completed our

study of the hexaphenylethane-triphenylmethyl radical system. Fluorescence

intensities as a function of excitation wavelength have been measured.

Results of these measurements indicate that the optimum excitation

wavelength for the hexaphenylethane system is at ~3300 A. Consequently,

tue N2 laser, with output at 3371 A is expected to be the best optical

_1 pump. However, the low value of the maximum gain of ~.02 cm requires

a longer excitation length, at fixed energy density, than realizable

with an N„ laser in order to overcome cavity losses and reach threshold.

Since the hcxaphenylethane fluorescence efficiency decreases as the k

excitation wavelength moves further towards the UV from ~3300 A, the

output of conventional flashlamp systems is poorly coupled to the

excitation requii ements of the hexaphenylethane molecule. Estimates

show that a flashlamp system with output power in excess of 10 MW

in the UV would be required to achieve .02 cm gain over a 15 cm

length, making such a pumping scheme highly unattractive.

In addition to hcxaphenylethane, the other molecules that

were synthesized in the early part of the program hav i been investigated,

In particular, fluorescence spectra and lifetimes of pentaphenylethane,

1-p-biphenyl-l, 1,2.,2-tetraphenylethane, and 12-12' bifluoradenyl have

been obtained. These systems however, exhibit no significant photo-

dissociation into excited state radicals, and therefore will not fulfill

the requiretients of a PDL active media.

II. THE PHOTODISSOCIATION DYE LASER CONCEPT

In its most general form, the photodissociatlon dye laser may be

understood as follows: A stable molecule, AB, in solution is optically

pumped to its first excited singlet state. The absorbed pump energy exceeds

the molecule's dissociation energy and two radicals are formed upon dissociation.

The excess energy is partitioned as electronic and vibrational excitation in

one or both of the radicals A* and B*. In particular, we consider radical A*

co be electronically excited. A partial population inversion is produced

between the first excited and the ground state of the radical. Laser action

occurs in A* and the resulting A* and B* ground state molecules are unstable

against recombination. The processes may be written as:

AB + Hump > (AB)*

(AB)* > A-* + B-

A-* > A- + hvlaser

A- + B« > A2 + 32 + AB

In the final recombination step, not all of the radicals form the original

molecule AB, but the dimeric forms A2 and B2 are also produced. Continuous

laser action would require replenishment of the starting material, AB.

Let us consider in more detail the special case where B - A so that

the starting molecule is a symmetrical dimer.

The general energy level scheme for such a photodissociatlon dye

laser molecule is shown schematically in Figure 1. The stable parent dimer,

having no unpaired electrons, exhibits the usual singlet and triplet level

structure. The ground and excited electronic singlet levels are denoted by

Do , Do ,. . . and the triplet levels are denoted by DT , DT,, . . . 0o ^ »g ^1

' I wpi •"">"-" ■'mmlm'.vlm!)-^v¥---*v''%^..- __,m.«———

- 5

The lowest electronic levels of the radicals, obtained from the

symmetric photodissociation of the dimer, are shown adjacent to the dimer

structure. The radical, having a single unpaired electron, exhibits a

doublet structure. In the figure the radical ground state is shown displaced

upward in energy from the dimer ground state by an amount equal to the dimer

dissociation energy.

The vibrational level spacing in both the dimer and radical ranges

between 150-1500 cm" while the rotational spacing ranges between 15-]50 cm" .

Therefore, as in conventional dye lasers, a quasicontinuum exists for each

electronic level comprised of the thermally broadened rotational and vibrational

love Is.

A characteristic of the photodissociation dye laser molecule is

that the Dc;, and D-p levels in the dimer lie above the dimer dissociation

energy. Consequently, upon optically pumping the dimer to Dg, the molecule

will undergo dissociation into two radicals. Dissociation may occur via

two possible paths. The first is directly from the optically pumped Ds

level. However, the Dg, level may, in principle, undergo a rapid intersystem

crossing to the Dj level. Dissociation of the dimor may occur from this

level.

Upon dissociation, the excitation energy is partitioned between

a manifold of levels in the quasi-continuum of both the excited and ground

state of the radical. The radical ground state initially has a negligible

population; consequently, a partial inversion in the radical can be produced.

The stimulated emission is tunable as in conventional dye lasers because of the

quasi-continuous distribution of the upper and ground levels. An interesting

point is that for radicals considered suitable for the photodissociation dye

■-:,

*

li on

A.,(De ) + hv 2 i>o pump

•* A2(DSl. V")

A2(DSl, v") ► A2(DSo, V')

A2(DSl. V) -

A2(DSl. V") -

* A2(DTo, v')

» A(R2, u'") + AtRj, V):

A (R2. V") ♦ Mtj, V") + hVlaser

A(R1) + A(R1) W

excitation of dimer

singlet relaxation

intersystem crossing

dissociation

laser action

recombination

Figure 1. Photodlssociation Dye Laser Molecule Structure and Kinetics

■ --—-. . - ——*-. —-~— ■->- " !

laser both the calculated and measured oscillator strengths for the f;rst

doublet-doublet electronic transition are always small. Thus, even though these

transitions are allowed by selection rule considerations, they exhibit a partial

"forbiddenness". Consequently, the radiative lifetime of the laser transitions

can be expected to typically be a factor of a hundred times longer than in conven-

tional dye lasers. This has been experimentally verified in some cases as will

be discussed later. The longer lived upper laser level can permit a larger

population density to be obtained and thereby allows the possibility of high

energy output.

Tue ability of the photodissociation dye laser to operate on a cw

basis requires, as usud, that the depletion rate of the lower laser level

exceed the radiative transition rate from the upper to the lower laser level.

The lower laser level is the radical ground state which is unstable to recom-

bination back to the original parent dimer. Thus for a steady-state population

inversion to be maintained, the radical recombination rate must be greater

than the laser transition radiative rate.

JU*ii*»-''>"-F«f—-»—r '"- ■■«l^_- __ _— _| _______________________

wmmmmammmmmmmmmmmammmmm

- 8 -

Ul- PROPERTIES OF THE SELECTED PPL MOLECULES

A class of organic molecules known as the hexaarylethanes

exhibit many of the prerequisites necessary for a PDL active media.

At the start of the program five compounds from this class were selected

and synthesized for further study. Details on the techniques used for

synthesis of these mrlecules were given in the first semiannual report ^l

What follows is an overview of the relevant properties of these materials

known prior to our experimental effort.

A. Hexaphenylethane

The hexaphenylethane molecule can be represented schematically

as;

©-r r®

Although the accepted structure is actually non-planar, with a quinoid-

type atomic arrangement, the above representation is adequate for the

purposes of this discussion.

I. Chemical Properties

a« Dissociation Into Free Radicals

It has been clearly established for some time that hexaphenyl-

ethane thermally dissociates in non-reactive solveuits to produce the

intensely colored triphenylmethyl radical^:

~w - ■ ' 1 " >~- 1 r— ■rvr-^r^"-

0

, ) c - c -(O

O

Hexaphenylethane (Colorless Solid)

-4 K « 2 x 10

(24', 0.10M in Benzene)

Triphenylmethyl Radical (Yellow in Solution)

The existence of the radical has been proven by a multiplicity of

analytical techniques. For example, cryoscopic molecular weight determinations

of such compounds in solution have shown that the apparent molecular weights

were well below that of the dimer. In addition, spectrophotometric measurements

have established that solutions of the dimer do not obey Beer's law: the

intensity of absorption increases with dilution as would be predicted for the

dissociation of a colorless compound into a colored radical. Finally, absolute

methods of radical detection - magnetic susceptibility and electron spin resonance

spectroscopy - have shown beyond doubt the thermal dissociation of hexaphenyl-

ethane into ground state triphenylmethyl radicals.

Two factors determine the position of the hexaphenylethane-triphenylmethyl

equilibrium: (a) Steric effects and (b) Radical stability.

(a) Steric Effects - This factor favors the formation of radicals

in two ways. First, there is a relief, upon dissociation, of the steric inter-

actions in the ethane. In essence, the central carbcu-carbon bond in the

hexaphenylethane is weakened by the steric repulsion of the aromatic rings

produced by interactions between the ortho-substituents. The bond weakening

can be experimentally verified by comparison of the bond-length and strength

of the central carbon-carbon bond in hexaphenylethane and the nonsterically-

hindered ethane:

- —

10

Hexaphenylethane

o

Bond Length 1.58 A

Bond Strength 11.5 kcal/mole

Ethane

1.54 A

85 kcal/mole

The second steric effect is typified by a sterlc hindrance to

recombination once a radical is formed.

(b) Radical Stability - The stability of the triphenylmethyl radical

is provided by a resonance stabilization effect resulting from the de1ocalization

of the free electron throughout the three aromatic rings of the structure,

b. Deleterious Reactions

(1) Disproportionation

A number of chemical properties act to complicate the use of hexaphenyl-

ethane as a PDL active media candidate. These properties arise from the reactivity

of the produced radicals. The first of these is their propensity towards

disproportionation. This transformation is promoted by heat or light and is

illustrated below:

[ o j

0

-"- T ' ■":—^—-

11 -

In essence, two molecules of the radical become reduced at the expense

of a third radical which is oxidized. For the triphenylmethyl radical,

the products are triphenylmethane and the dehydro-dimer of 9-phenylfluorene.

Measurements of the effect of this photochemical or thermal degradation

on the use of hexaphenylethane as a laser media were presented in the

(3) second semiannual report .

(2) Addition Reactions

The second deleterious reaction path which occurs when radicals are

produced is the addition reaction. This involves the rapid absorption of

atmospheric oxygen to form colorless triphenylmethyl peroxides:

2 /

c ,- c •

v_/ o-o

As the radicals react, more dissociation must occur in order to maintain the

dimer-radical equilibrium constant. In this way, a continuous depletion of

both the radical and dimer concentration occurs until all the hexaphenylethane

in solution has reacted to form the peroxide precipitate. As pointed out in

the first semiannual report , careful preparation under vacuum conditions

and subsequent storage and handling under a nitrogen atmosphere yield solutions

that are quite stable with respect to such reactions. Spe-trophotometric

measurements of the radical concentration over a period of time in solution«

we have prepared indicate a decrease of only several percent per week; and

this decrease is probably due primarily to thermal disproportionation.

12

II. Spectroscopic Properties

The reported major bands in the electronic absorption spectra of

hexaphenyletbane and the ti . enylmethyl radical are shown below along with

(4) the measured extinction coefficients

Hexaphenyle thane

o

^max = 3150 A (using KBr pellet technique)

o

^max = 3130 A (dissolved in cyclohexane)

Triphenylmethyl Radical

^max = 3^50 A, e = 11,000 O

^max = 5100 A, e = 210

(both in cyclohexane)

The major absorption bands can be associated with electronic

transitions. The resulting dimer and radical energy level structure is

shown in Figure 2. The first excited singlet level of the dimer, Dg,,

is ~ 31,750 cm above the ground state. The activation energy for dissociation

is ~ 7000 cm"1( while the dissociation energy is ~4000 cm . The

ground vibrational level of the first excited electronic state in the

triphenylmethyl radical lies at 19,410 cm and the second excited electronic

level is at ~ 29,000 cm above the ground state.

The mirror image symmetry of the absorption and fluorescence bands

ts illustrated in Figure 3. The spectra were originally reported by Lewis

et al. W and were measured by suddenly c .ng a solution of hexaphenylethane

in EPA (5 parts ether, 5 parts isopentane, parts ethanol by volume) to

liquid N2 temperature (-190oC). It was reported that rapid cooling preserved

the highly colored radicals in the clear EPA glass, and that under these

conditions, there was no evidence of disproportionation. Unfortunately, no

details regarding the measurement techniques used in taking the fluorescence

data were pre8ented--nor was the excitation source or wavelength described.

13 -

t 8

m XI

o

30 - >

20

10

5100A

Hexaphenylethane Triphenylmethyl Dimer Radical

E, = Activation Energy - ^7000 cm-1 A

E = Dissociation Energy « ^4000 cm-1 D

Figure 2. Energy Level Structure of the Hexaphenylethane- Triphenylmethyl System.

14

30

20

10

ieoo I 6000

18001 5500

_L 2000 5000

• " 22001 cm^V 4700 4 500 A X

Figure 3. Mirror Symmetry of Absorption and Fluorescence Bands of the Triphenylmethyl Radical (in EPA mixed solvent at -190oC). Ref. (7).

-- "■'■■■■

15

More recently. Okamura et al. (7) performed time resolved fluorescence

measurements on the triphenylmethyl and other methyl-substituted radicals

which were trapped in rigid solvents at low temperature. The triphenylmethyl

radical was prepared by the photolysis of triphenylmethare molecules at -190oC

in a quartz cell using a low pressure mercury lamp. An N2 laser emitting

a 10 nsec, 40 kw peak power pulse at 3371 A was used as the excitation source.

The fluorescence decay consisted of a single exponential with the measured

decay times being 280 nsec with ethyl alcohol as solvent, and 330 nsec using

isopentane as the solvent. The very long fluorescence lifetimes were taken as

evidence that the first doublet-doublet electronic transition in the radical

has a forbidden character, although such transitions are allowed by the usual

selection rule considerations. The possibility that the observed lifetimes

were actually longer than the natural lifetimes because of complex formation

of the excited state with the solvent, was ruled out by the fact that the

difference of measured lifetimes in polar and nonpolar solvents wa-* not

appreciable.

H. Other Hexaarylethane Derivatives

The four other molecules that were synthesized have the

advantage of being completely stable at room temperature with respect

to dissociation into radicals. They all have strong absorption features

in the UV. Their structures and UV absorption data are presented

in the first semiannual report' *«

■N

16

Three of the four molecules: sesquixanthydril. pentaphenyl-

ethane, and 1-p-blphenyl-l,1,2,2-tetraphenylethane are known to dissociate

into colored radicals upon heating to ~100oC, indicating the weak

bonding inherent in this group of molecules. Although no thermal or photo-

chemical data with regard to radical production is available for 12-12'

bifluoradenyl, its structure is sufficiently similar to hexaphenylethane

to warrant its inclusion as a possible PDL material.

Unfortunately, solubility problems have precluded further

study of sesquixanthydril as a potential PDL molecule. A spectroscopic

study and evaluation of the three remaining molecules is presented

in this report.

"•»",«-' "'■■■-——

■äKaüfiu^^M&äiSlb

- 17 -

IV. EXPERIMENTAL EFFORT AND RESULTS

A. llexapheny le thane

Aside '.rom the synthesis of five potential PDL materials, the

major part of the fv-erimental effort reported in the previous semi-

annual reports has been directed towards obtaining data that is relevant

to assessing the possibility of laser action in the hexaphenylethane-

triphenylmethyl radical system.

On the positive side, photodissociation of the hexaphenylethane

dimcr into excited state triphenylmethyl radicals has been demonstrated.

Optical gains of -.02 cm have been determined from absolute intensity

measurements utilizing N2 laser excitation (assuming the lower laser

level to be unpopulated). Fluorescence lifetimes of the first excited

triphenylmethyl radical level (the upper laser level) of ~200 ns have

been measured at low temperature (-80 C).

On the negative side, the room temperature fluorescence lifetime

is short (-15 nsec), the material must be handled in an oxygen-frse

environment because of the extreme reactivity of the thermally-generated

radicals, and photochemical stability is poor. In addition, the effective

quantum yield of fluorescence measured at the peak of the UV absorption

band (-3350 A) is low. (These measurements were performed using an

N laser whose output (3371 A) essential coincides with the peak of

the absorption band.) For every 100 3371 A photons absorbed by the

hexaphenylethane, only one photon comes out as triphenylmethyl radical

fluorescence, giving an effective quantum yield of 1%. For further

details on these measurements the reader is referred to the previous

(1.3) semiannual reports

- 18 -

The first topic that has been addressed during the present

reporting period is that of the dependence of the hexaphenylethane fluorescence

intensity on the excitation wavelength. This study was motivated by the

fact that although the peak of the hexaphenylethane-triphenylmethyl UV

absorption curve lies very close to the 3371 A nitrogen laser wavelength,

the cross section for photodissociation into excited state radicals may

peak at a different wavelength, and as a result, larger fluorescence

yields may be obtainable at other wavelengths even though the UV absorption

is less. In addition, a knowledge of how the triphenylmethyl radical

fluorescence intensity varies as a function of excitation wavelength

is necessary in order to make an evaluation of different possible pumping

mechanisms (e.g. laser vs flashlamp).

A frequency-doubled dye laser was used to provide a tunable

UV source for these measurements. The peak of the fluorescent emission

(5200 A) from the hexaphenylethane solution was passed through a mono-

chromator and detected by a 1P21 photomultiplier tube. The pulsed output

was time-averaged with a PAR 160 boxcar integrator. A Molectron J3-05

joulemeter was used to measure the incident laser intensity at each

wavelength. Figure 4 shows the results of this series of measurements.

The plotted ratio represents the hexaphenylethane fluorescence intensity

normalized to the incident excitation intensity. If a comparison of this

curve to the hexaphenylethane UV absorption curve (Figure 5) is made

one sees that they are quite similar in shape.

From the point of view of assessing an appropriate optical

pump source for hexaphenylethane, thepa findings lead to some Important

conclusions. Optical pumping by a N2 laser looks attractive for two

reasons. First of all, the N2 laser output of 3371 A falls close to

•^—— ^S ?>'»NW„

- 19

o o o

o 00

00

o o

o o

o o

O O n to

o O CM 1*1

o o

w

o o

o o o

o o o

Q

+ + 4- + -f-

• 'S CM

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21

the peak of the UV absorption curve. Secondly, optical pumping at this

wavelength not only results in the maximum absorption, but in the

largest fluorescence quantum yield as well.

An attempt at achieving laser action in a solution of hexaphenyl- ■

ethane in isooctane has been made utilizing N2 laser pumping. The 300 kw,

8 ns output of our N2 laser was focused onto a quartz cell equipped with

brewster angle windows. Two highly reflective dielectric coated mirrors

served to iorm a resonant cavity. However, no laser action was attained

from this configuration. Unfortunately, the excitation length one

can achieve with the ^ laser for a given power density is essentially

limited by the beam geometry. In our case, using a cylindrical lens

to Conn a line image, the effective excitation length is ^2.5 cm.

Taking our estimated value for the gain coefficient, of .02 cm"^,

this means that the single pass gain through the hexaphenylethane solution

is at most 57,. and this is not sufficient to overcome other cavity

losses.

Xenon flashlamp pumping has been considered as a means of exciting

a longer excitation length. In the N2 laser pumping experiment, ~10 MW/cc

are absorbed by the hexaphenylethane solution. In order to maintain this

absorbed power density along a 15 cm length of active region would require

a flashlamp output power equivalent to ~2 MW at 3371 A. The fact that the ■

flashlamp output is broadband and that both the hexaphenylethane absorption

and triphenylmethyl radical fluorescence falls off rapidly as one moves

further into the UV from 3371 A means that the flashlamp output must be

O

on the order of ~10 MW in the range 2800-3600 A in order to be equivalent

to the Nj laser pump in exciting the hexaphenylethane solution. These

large pumping requirements have led us to the conclusion that Xenon flashlamp

pumping of hexaphenylethane would be highly impractical.

——•_ ■ ■■■"" -"•• ■ '■' —^^ ' . ■ i 1 —.-,,_„_ ,•_—r—i-mr

- 22 -

B« Other PPL Molecules

In addition to the work on hexaphenylethane, fluorescence

spectra have been obtained during this reporting period for the other

PDL molecules that were synthesized at the start of the program. The

excitation source for all the spectra discussed below is a frequency

doubled dye laser tuned to 2600 A. All the molecules described

below are strongly absorbing at this wavelength.

(a) Pentaphenylethane - Breaking of the central carbon-carbon

bond results in the production of both triphenylmethyl and diphenylmethyl

radicals. As mentioned previously thermal production of such radicals

has been noted historically as color changes in heated solutions of

pentaphenylethane. The fluorescence spectrum shown in Figure 6

does not show any features in the visible that can be associated with

production of these radicals in an excited electronic state. Rather,

a broad feature in the UV dominates the spectrum and is probably due

to fluorescence of the pentaphenylethane dimer structure. We conclude

that UV excitation of pentaphenylethane does not result in any significant

photolytic production of electronically excited radicals.

(b) l-p-biphenyl-l,l,2,2-tetraphenylethane - For this molecule

as well, thermal dissociation into radicals has been observed by noting

that clear solutions turn colored upon heating due to the visible

absorption spectrum of the radicals. However, the fluorescence spectra

we have taken (Figure 7) do not show any corresponding visible fluorescence

that would be indicative of the formation of radicals in an electronically

excited state. The weak fluorescence that is observed in the UV can

be attributed to the dimer.

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23

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25

(c) 12-12' Bifluoradenyl - No evidence for thermal dissociation

into radicals has been reported in the literature for this structure.

The fluorescence spectra we have observed (Figure 8) indicate several

peaks in the blue and UV. However, the fluorescence intensity is

extremely weak.

The fluorescence lifetime for all these structures has been

measured. The room temperature lifetimes are all ~10 ns with no

observable change as the temperature of the medium is lowered to

--lOO'C.

(d) We have investigated one other molecule that was not one

of the original PDL molecules synthesized at the start of the program.

This molecule is triphenylmethylchloride:

Oi- Cl Irradiation of a solution containing this molecule with 2600 A light

results in triphenylinethyl radical fluorescence. Photodissociatlon

resulting in excited radical states is clearly seen to occur in this

material. However, the fluorescence quantum yield is less than 1%,

and therefore this molecule is not suitable as a PDL active media.

mamm

2h

M u 00 c

T-l

0) >

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u

s 0) >>■

"V cu

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x>

Ü e 3 tr

tu

to

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oo

27 -

V. CONCLUSIONS

During this reporting period we have completed our measurements

on the potential PDL molecules that were selected from the hexaarylethane

group, and have evaluated their use as active media for the PDL scheme.

Fluorescence intensity measurements as a function of excitation

wavelength have been made on the hexaphenylethane-triphenylmethyl radical

system. These measurements indicate that the optimum optical pump

source for the system is the N2 laser. However, because the

maximum gain for the system is low (-.02 cm"1), the length of the active

media that the N2 laser can excite is too short to overcome the single

pass cavity losses.

The remaining hexaarylethane molecules synthesized at the

start of the program have been öcudied as well. Their measured fluorescence

lifetimes are -10 ns in all cases. Their fluorescence spectra indicate

that these molecules do not undergo appreciable photodlssociation upon

UV excitation (2600 A) into excited electronically radicals. They

are, therefore, not suitable as PDL active media.

Further progress inlhia program will involve synthesis of new

candidates to fulfill the requirements of the PDL scheme. Possible

choices of new groups of organic molecules to consider are the dilactones,

alkoxytetraarylethanes, and tetraphenylallyl molecules. All these

groups have been reported to reversibly dissociate into radicals as

a function of temperature.

- 28 -

VI. REFERENCES

1. A. Kasdan, "Photodlssociation Dye Laser," Semiannual Technical Report, Contract No. N00014-73-C0048, April, 1975.

2. A summary of the background information, relevant literature, and pertinent lead references tor hexaphenylethane are included in the following sources.

a. A. R. Forrester, J. M. Hay and R. H. Thompson, "Organic Chemistry of Stable Free Radicals," Academic Press, Inc., New York (1968).

b. C. J. M. Stirling, "Radicals in Organic Chemistry," Oldbourne Press, London (1965).

c. Glenn H. Brown, Ed., "Photochromism," Techniques of Chemistry, Vol. Ill, Wiley-Interscience, New York (1971).

d. Henry Oilman, Ed., "Organic Chemistry--An Advanced Treatise," Volume 1, 2nd Ed., John Wiley and Sons, Inc., New York, (1963), pp. 581-630.

3. A. Kasdan, "Photodlssociation Dye Laser," Semiannual Technical Report, Contract No. N00014-73-C0048, October, 1975.

4. Lankamp. Nauta, and MacLean, Tetra. Letts 2, 249 (1968).

5. Ziegler, Orth, and Weber, Ann. 504, 131 (1933).

6. Ziegler and Ewald, Ann. 473, 169 (1929).

7. G. N. Lewis, D. Lipkin, and T. T. Magel, T. Am. Chem. Soc. 66, 1579 (1944). ~

8. T. Okatnura, K. Obi, and I. Tanaka, Chem. Phys. Letts. 20, 90 (1973).

■

.

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Distribution List for ARPA Order #1806/1807 Unclassified Contracts

Science Applications Inc. PO Box 128 Ann Arbor, MI 48103 Attn: R. E. Meredith

Science Applications Inc. 6 Preston Oourt Bedford, MA 01703 Attn: R. Greenberg

Science Applications Inc. PO Box 2351 La ITolla, CA 92037 Attn: Dr. John Asmus

Systems, Science and Software PO Box 1620 LaJolla, CA 92037 Attn: Alan F. Klein

Systems Consultants, Inc. 1050 31st St. NW Washington, DC 20007 Attn: Dr. R. B. Keller

Thiokol Chomical Corp. WASATCH Div. PO Box 524 Brigham City, UT 84302 Attn: Mr. J, E. Hansen

TRW Systems Group One Space Park Bldg. R-l RM 1050 Redondo Beach, CA 90278 Attn: Mr. Norman Campbell

United Aircraft Res. Labs 400 Main Street East Hartford, CT 06108 Attn: Mr. G. H. McLafferty

Mr. Albert Angelbeck

United Aircraft Corp. Pratt and Whitney Acft. Div. Florida R&D Center West Palm Beach, FL 33^02 Attn: Dr. R. A. Schmidtke

Mr. Ed Pinsley

1 copy

1 copy

1 copy

1 copy

1 copy

1 copy

1 copy

3 copies 1 copy

1 copy 2 copies

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Distribution List for ARPA #0rder 1806/1807 Unclassified Contracts

VARIA] Associates EWAC Dlv 301 Industrial Way San Carlos, CA 94.070 Attn: Mr. Jack Quinn 1 copy

Vought Systems Div. LTV Aerospace Corp. TO Box 5907 Dallas, TX 75222 Attn: Mr. F. G. Simpson

Mail Station 25042 1 copy

WfGtinghouse Electric Corp Dpfonsr and Space Center Friendship International Airport - Box 74.6 Baltirnor- , MD 21203 Attn: Mr. W. F. List 3 coplßs

Westinghnuse Res. Labs Beulah Road, Churchill Boro Pittsburgh, PA 1523r' Attn: Dr. F. P. Riedel 1

Mr. R. L. Hundstad l co^

United Aircraft Company Research Laboratories Attn: A. J. DeMaria East Hartford, CT 06108 1

Airborne Instruments Laboratory Attn: F. Pace Walt Whitorian Road Melville, NY 11746 1 copy

Stanford Research Institute Menlo Park, CA 94025 Attn: Dr. F. T. Smith 1 copy

General Electric R&D Center Schenectady, NY 12305 Attn: Dr. Donald White T „nntr 1 copy

Cleveland State University Cleveland, OH 4^115 Attn: Dean Jack Soules T ,, 1 copy

Esso Research and Qigineering Co. Attn: D. Grafstein PO Box 8 Linden, NJ 07036 , 1 copy

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Distribution List for ARPA Ordar ,fT806/l807 Unclaesifled Contracts

University of Maryland Department of Physics and Astronony Attn: D. Curry College Park, MD 20742 1 copy

Sylvania Electric Products, Inc. Attn: L. M, Osterlnk 100 Fergeson Drive Mountain View, CA 94040 1 copy

North American Rockwell Corp, Autonetics Division Attn: R. Gudnundsen 3370 Miraloma Avenue Anaheim, CA 92803 1 copy

Massachusetts Institute of Technology Att"»; Prof. A. Javan 77 Massachusetts Avenue Cajnbridge, MA 02138 1 copy